The present invention relates to a discharge lamp ballast in which the discharge lamp is dimmed by rectangular wave voltages, for example a discharge lamp ballast suitable for dimming external electrode type discharge lamps, which are used as backlights for liquid crystal displays.
As backlights in, for example, liquid crystal displays, there has been intensive research into rare gas discharge lamp devices of an external electrode type that use dielectric barrier discharge. The basic reason for this is that rare gas discharge lamp devices do not use mercury, and therefore do not incur the reduced luminous efficiency that accompanies the rise in temperature of mercury. They are also preferable for environmental reasons.
The lamp operation using dielectric barrier discharge depends on an effect in which a gas discharge is caused with the high voltage generated when charging the dielectric layer by application of a driving voltage and then inverting the driving voltage. For this purpose, a high frequency rectangular wave voltage is used as the driving voltage. An example of a discharge lamp ballast that dims the discharge lamp with a rectangular wave voltage is disclosed in JP 2001-267093A. As shown in FIG. 10, this discharge lamp ballast includes a DC power source 33, a rectangular wave generating circuit 34, a dimmer control circuit 35, a synchronization circuit 36, a pulse transformer 37, and a discharge lamp 38 having one external electrode.
The rectangular wave generating circuit 34 shapes the DC voltage supplied from the DC power source 33 into, as shown as a waveform (a) in FIG. 11, a high frequency rectangular wave signal Vp of predetermined frequency. The dimmer control circuit 35 outputs a dimmer signal Vdim with the PWM waveform (b) in FIG. 11, and supplies it to the synchronization circuit 36. From the high-frequency rectangular wave signal Vp, the synchronization circuit 36 extracts, as a driving signal VL, a burst waveform corresponding to the ON period of the dimmer signal Vdim (c) in FIG. 11. Based on the driving signal VL, a rectangular wave voltage for driving the lamp, which has been raised with the pulse transformer 37, is applied to the discharge lamp 38, and lights the discharge lamp 38. In this configuration, dimming is performed by changing the duty factor of the dimmer signal Vdim.
However, the phases of the dimmer signal Vdim and the rectangular wave signal Vp are not always in agreement. And for that reason, when the extraction of the rectangular wave signal Vp is turned on/off at the same time that the dimmer signal Vdim is turned on/off, the extracted driving signal VL sometimes includes rectangular pulses of incomplete widths at the beginning or end of sequences. When using dielectric barrier discharge for lamp operation, and when the applied rectangular wave is not wide enough, then the amount of charge supplied to the dielectric layer may be insufficient, so that the discharge lamp 38 will not emit light. When this non-emission occurs under a condition of the dimmer ratio of 10% or less, this causes a flickering sensation. Therefore, in order to suppress flickering under a condition of a high dimming ratio, it is necessary that all the rectangular pulses contained in the driving signal VL maintain the waveform of the rectangular wave signal Vp.
The ballast disclosed in JP 2002-75684A, for example, meets this condition. With this device, the dimmer signal is digitized by A/D conversion, and the number of generated driving signal pulses is controlled based on that digital value. Since the output of the rectangular waveform of the driving signal VL is controlled digitally, rectangular pulses of incomplete widths are not output. However, with this device, to change the frequency of the dimmer signal (the dimmer frequency), it is necessary to change the entire circuit configuration. Therefore, in practical terms, the frequency is fixed. Conventionally, when discharge lamps are used as the backlights for liquid crystal displays, in order to prevent the interference of noise with the liquid crystal display, it is preferable to set the dimmer frequency to conform to the driving circuit of the liquid crystal. Therefore, the discharge lamp disclosed in JP 2002-75684A, in which the dimmer frequency is fixed, is lacking in versatility. Moreover, because it uses a microcomputer, the ballast is complex and therefore expensive.
In contrast to this, the ballast in JP 2001-267093A has a freely adjustable dimmer frequency, a simple configuration, and is inexpensive. Moreover, JP 2001-267093A describes a configuration for maintaining an appropriate waveform for the rectangular pulses of the driving signal VL. With this ballast, the timing for extracting the rectangular wave signal Vp is controlled as follows via the synchronization circuit 36 shown in FIG. 10. First, at the beginning of the driving signal VL, the extraction of the rectangular wave signal Vp begins at a rising of the rectangular wave signal Vp during the period in which the dimmer signal Vdim is on. At the end of the driving signal VL, even when dimmer signal Vdim is turned off, the extraction of the rectangular wave signal Vp continues until the falling of rectangular wave signal Vp. Therefore, as shown by the waveform (c) in FIG. 11, only driving signals VL that are composed of complete waveforms are input to the pulse transformer 37.
However, there are the following problems with the discharge lamp ballast in JP 2001-267003A: If the frequency of the rectangular wave signal Vp is not an integer multiple of the dimmer signal Vdim, this may lead to the problem of the phase difference between the two waveforms changing with time. This is explained with reference to FIG. 12.
In FIG. 12, a waveform (a) illustrates the dimmer signal Vdim, and waveforms (b) and (c) illustrate two rectangular wave signals Vp having different phase differences with respect to the dimmer signal Vdim. Waveforms (d) and (e) show the driving signals VL that have been generated from the rectangular wave signals Vp (b) and (c), respectively. In the case of the waveform (b), the ON period of the dimmer signal Vdim starts while the rectangular wave signal Vp is ON. Consequently, the driving signal VL is output after the next ON timing of the rectangular wave signal Vp. On the other hand, in the case of the waveform (c), both the dimmer signal Vdim and the rectangular wave signal Vp are turned on in synchronization. Consequently, the driving signal VL is output at the same time as the start of the ON period of the dimmer signal Vdim. On the other hand, after the ON period of the dimmer signal Vdim has been completed, in both cases of the waveforms (b) and (c), the ON period of the rectangular wave signal Vp has not yet completed, so that it is extracted as the driving signal VL until the falling of the rectangular wave signal Vp. As a result, the driving signal VL (e) in FIG. 12 contains one pulse more (the hatched pulse in the figure) than in the case of the waveform (d).
Thus, if the number of pulses of the driving signal VL for a dimmer signal Vdim of the same duty factor changes, then this may cause flickering in the discharge lamp 38. In particular when the dimming ratio is high, and consequently the duty factor of the dimmer signal Vdim is low (the ON period is short), then the number of pulses of the driving signal VL included in the ON period of the dimmer signal Vdim is small, so that there is a large change in the emitted luminance depending on whether there is one pulse more or less, and the extent of flickering becomes large.
In addition to this problem, when the AC rectangular wave voltage produced from the driving signal VL is stepped up with a pulse transformer, the following problems occur: When the rectangular wave voltage applied to the pulse transformer becomes zero at the end of the driving signal VL, ringing occurs due to uncontrolled voltage oscillations. Due to this ringing, the waveform of the output of the pulse transformer corresponding to the end of the driving signal VL becomes chaotic, so that erroneous discharges of the discharge lamp occur, which cause flickering. This problem cannot be solved by optimizing the generation of the driving signal VL from the rectangular wave signal Vp. The effect of this is significant in particular for high dimming ratios.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a discharge lamp ballast that operates a discharge lamp with a rectangular wave voltage, with which the dimming frequency can be set freely, and flickering under high dimming ratios can be suppressed.
A discharge lamp ballast in accordance with the present invention includes a rectangular wave generating circuit that generates a rectangular wave signal of a predetermined frequency; a driving signal generating circuit into which the rectangular wave signal and a dimmer signal of a PWM waveform of a frequency lower than that of the rectangular wave signal are input, and which outputs, as a driving signal, a signal obtained by extracting the rectangular wave signal for a period corresponding to a period in which the dimmer signal is on; and a pulse transformer in which a rectangular wave voltage based on the driving signal is applied to the primary side, and the rectangular wave voltage is stepped up and applied to a discharge lamp.
In order to solve the above-described problems, a discharge lamp ballast according to a first aspect of the present invention is provided with a rectangular wave reset circuit that resets the operation of generating the rectangular wave signal with the rectangular wave generating circuit at a rising of the dimmer signal. The driving signal generating circuit extracts the rectangular wave signal for a period starting from when the dimmer signal is turned on, until a time at which the dimmer signal and the rectangular wave signal are both off.
A discharge lamp ballast according to a second aspect of the present invention is provided with an output reset circuit that inputs a pulse voltage in phase with the rectangular wave AC voltage to the pulse transformer immediately after the end of the rectangular wave AC voltage in response to the completion of the ON period of the dimmer signal, the secondary side output of the pulse transformer being attenuated by application of the pulse voltage.